1. Field of the Invention
The present invention relates to an image forming apparatus for copying machines, printers, facsimiles, etc. which forms images by an electrophotographic system, an electrostatic recording system, etc.
2. Description of the Related Art
Electrophotographic full-color image forming apparatus transfers a developed image (toner image) from a photosensitive member, which is a first image bearing member that carries it, to an intermediate transfer member, which is a second image bearing member, at a primary transfer section. Furthermore, there are many commercialized products that employ an intermediate transfer belt system that transfers toner images in a plurality of colors superposed on the intermediate transfer member onto a transfer material, such as paper, at a secondary transfer section.
FIG. 9 is a schematic configuration diagram showing an example of an image forming apparatus using a conventional intermediate transfer belt system.
This image forming apparatus is configured to form toner images at a plurality of image forming sections provided along an intermediate transfer belt 8 on the basis of image input data. The image forming apparatus is configured to form electrostatic latent images on individual photosensitive drums 2, develop single-color toner images of the electrostatic latent images to form single-color toner images, superpose the single-color toner images formed at the individual image forming sections on the intermediate transfer belt 8 to form a multiple (multicolor) toner image, transfer the multiple toner image onto a transfer material, and fix the multiple toner image on the transfer material with a fixing device 21.
Here, drum-shaped photosensitive members (photosensitive drums) 2 (2a, 2b, 2c, and 2d) are arranged in a line at the color-image forming sections.
There are charging rollers 7 (7a, 7b, 7c, and 7d), exposure devices 1 (1a, 1b, 1c, and 1d), developing units 3 (3a, 3b, 3c, and 3d), photosensitive-drum cleaning units 5 (5a, 5b, 5c, and 5d) around the photosensitive drums 2, respectively. The photosensitive drums 2 corresponding to the individual colors are rotationally driven by driving units (not shown) at a predetermined processing speed.
The lower parts of the color photosensitive drums 2 are in contact with primary transfer rollers 4 (4a, 4b, 4c, and 4d), which are transfer members, with the endless intermediate transfer belt 8 serving as the second image bearing member therebetween, at individual primary-transfer nip portions.
The intermediate transfer belt 8 is stretched over a secondary-transfer counter roller 15 serving also as a driving roller, a tension roller 9, and a stretching roller 11 and is rotated in the direction of the arrow.
A transfer material P (not shown) that is a sheet conveyed from a paper cassette 20 is introduced from a registration roller pair 13 by a guide before secondary transfer 16 to a secondary-transfer nip portion formed by a secondary transfer roller 10 and the intermediate transfer belt 8. The toner image formed on the intermediate transfer belt 8 is transferred onto the transfer material P and is fixed by application of pressure and heat by the fixing device 21. Thus, the toner image can be formed on the transfer material P.
On the other hand, a toner image (waste toner) on the intermediate transfer belt 8 which cannot be transferred to the transfer material P at the secondary-transfer nip portion is cleaned by an intermediate-transfer-member cleaner 12.
With such a configuration, the secondary transfer roller 10 is always in contact with the intermediate transfer belt 8, and therefore, it is sometimes stained with the toner on the intermediate transfer belt 8. If the secondary transfer roller 10 is stained, the stain on the secondary transfer roller 10 stains the back of the transfer material P (a surface opposite to the transfer surface), that is, the back of the transfer material P is prone to be stained.
As a solution to those problems, for example, Japanese Patent Laid-Open No. 2001-356619 discloses an apparatus in which a waste-toner box for collecting removed toner is installed below the secondary transfer roller 10. This allows the secondary transfer roller 10 to be reliably cleansed without an influence of the intermediate transfer belt 8, thereby permitting high-quality printing with less back stain of the transfer material.
However, it is difficult for the system disclosed in Japanese Patent Laid-Open No. 2001-356619 to make the apparatus compact by a volume corresponding to the waste-toner box for the secondary transfer roller. In particular, with an image forming apparatus having a marginless print mode in which a toner image is formed to the edge of a transfer material, a large amount of toner adheres to the secondary transfer roller, which significantly increases the amount of toner to be collected. Therefore, in the system disclosed in Japanese Patent Laid-Open No. 2001-356619, the waste-toner box for the secondary transfer roller is increased, thus making it more difficult to minimize the apparatus. Even if a changeable waste-toner box for the secondary transfer roller is used to achieve minimization of the apparatus, a problem of increasing the frequency of replacement of the waste-toner box occurs.